Geddes, Tacker, and colleagues have pointed out that transthoracic ventricular defibrillation is seldom achieved in human patients weighing over 10 kg and, on the basis of laboratory work with large animals, have suggested that insufficient energy capabilities of presently available commercial defibrillators may be the reasom for our failure to salvage these larger patients. If defibrillators with higher energy capabilities are to be designed, it is essential that waveform dependency considerations be correctly incorporated into the design decision making process. In a long series of papers from 1963 through 1971, our group has detailed the relationship between waveform and effectiveness of transthoracic defibrillation in medium sized and large dogs. In the work outlined here, we propose to test the hypothesis that the comprehensive body of data available from our work with dogs can be extrapolated to 90-110 kg calves by a simple procedure involving current scaling. Our experimental study will utilize a new hydrogen thyratron defibrillator with a peak current capability of about 400 amperes and a power capability in the neighborhood of 4 megawatts. Transthoracic defibrillation studies in calves will be carried out with rectangular, untruncated exponential, and truncated exponential waveforms. We believe that the explicit data derived in this study will prove invaluable in the rational design of defibrillators for clinical use.